CVD (or VPE) takes place in an apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically about 10 KPa. Ammonia and the species providing one or more Group III elements to be used in epitaxial growth are supplied substantially parallel to the surface of a substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to and flowing across the substrate surface. It is in this gaseous boundary layer that decomposition to form nitrogen and the other elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria. To improve epitaxial growth of gallium nitride using the CVD process, it has been proposed in Appl. Phys. Lett. 58, (18), 6 May 1991, pages 2021-2023, to use a sub-flow of inactive gas perpendicular to the substrate for the purpose of changing the direction of the main flow of gaseous reactants across the substrate so as to bring the reactant gas into contact with the substrate. However, even such a modified CVD technique requires high growth temperatures to promote a high quality epitaxial deposit with all the attendant disadvantages of high growth temperatures, such as liberation of unwanted contaminants and increased evaporation of deposited material from the substrate.
Another form of CVD (or VPE) apparatus for the epitaxial growth of gallium nitride is disclosed in J. Electrochem. Soc., 125, (1978), pages 1161-1169, which discloses the catalytic activity of gallium and gallium nitride on the decomposition of ammonia. In such apparatus, hydrogen chloride is passed over gallium contained in a boat in a source zone of a furnace so as to produce gallium chloride which is passed through a furnace centre zone at which ammonia is introduced. The gallium chloride and ammonia then pass into a deposition zone of the furnace at which the substrate is mounted so that epitaxial growth can take place by vapour phase epitaxy with the gallium or gallium nitride assisting in the vapour phase decomposition of the ammonia to form nitrogen.
An appreciation of the basic differences between CVD (or VPE) and MBE will be well understood by those skilled in the art. Essentially, in the case of MBE as applied to the GaN system, an ultra-high vacuum (UHV) environment, typically about 1.times.10.sup.-3 Pa is used with a relatively low substrate temperature, typically about 750.degree. C., and with ammonia or another nitrogen precursor being supplied to the MBE chamber by means of a supply conduit and species providing gallium and, possibly, indium and/or aluminium being supplied from appropriate sources within heated effusion cells fitted with controllable shutters to control the amounts of the species supplied into the MBE chamber during the epitaxial growth period. The shutter-controlled outlets from the effusion cells and the nitrogen supply conduit face the surface of the substrate upon which epitaxial growth is to take place. The ammonia and the species supplied from the effusion cells travel across the MBE chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.
Various proposals have been made for improving epitaxial growth of gallium nitride by MBE. For example, S. Strite et al, in section A 2.4, Handbook of Thin Film Process Technology, edited by D. A. Glocker et al, Institute of Physics (1995), disclose plasma-enhanced MBE of gallium nitride so as to convert molecular nitrogen into its atomic form necessary for growth. However, only small useful ratios of nitrogen to gallium are possible using plasma sources. M. Kamp et al in Proceedings of Materials Research Society fall meeting, Boston USA (1995), pages 1-4, and M. Kamp et al in Proceedings of Topical Work Shop on III-V Nitrides, Nagoya, Japan (1995), disclose a technique referred to as On-Surface Cracking (OSC) for inducing thermal cracking of ammonia using what is essentially a typical MBE reactor and system for introduction of ammonia, but where the ammonia introduction nozzle is kept at a relatively low temperature whilst employing a relatively high temperature (typically 800.degree. C.) at the substrate to achieve the best gallium nitride crystal properties.
GB-A-2066299 discloses a method for growing doped III-V alloy layers by molecular beam epitaxy where the Group III species (Ga and/or In) and the Group V species (As and/or P) are supplied into a vacuum chamber by respective effusion cells which are arranged to discharge into the interior of a shroud which is disposed within the vacuum chamber and which is fitted with a shutter between the cells and the substrate on which epitaxial growth is to take place. The cells for the Group V species are longer than the cells for the Group III species and have two heating zones.
EP-A-0633331 and EP-A-0565455 disclose processes for preparing a high crystallinity oxide superconductor film (eg a Y--Ba--Cu--O type film) by molecular beam epitaxy. In such process, the metal species are supplied from effusion cells to the substrate through an apertured partitioning plate whilst oxygen and ozone are supplied in the vicinity of the substrate via gas-introducing nozzle.
EP-A-0540304 discloses a method for the manufacture of a Group II-VI compound semiconductor containing nitrogen as an impurity (dopant). The nitrogen is supplied in relatively small amounts using a supply which is remote from the substrate, which produces nitrogen excitation species (eg N.sup.+, N.sub.2.sup.+ and N) from gaseous nitrogen, and which discharges these species as a beam towards the substrate.
Patent Abstracts of Japan, Vol 13, No 592 [C-671] (JP-A-1-249692) discloses a molecular beam epitaxy device in which a Group V evaporation source, such as an As evaporation source, is disposed nearer to the substrate than the other evaporation sources to prevent wasteful scattering of the molecular beam.
U.S. Pat. No. 5,637,146, published on 10 Jun. 1997, discloses a method for the growth of semiconducting Group III nitrides, such as GaN, InN and AlN and their alloys, by molecular beam epitaxy. In such method, the source of nitrogen is atomic nitrogen produced by dissociation of high purity nitrogen using an RF excited plasma source or a nitrogen thermal cracker. The source of atomic nitrogen is separated from a substrate manipulator by a distance of less than 15 cm to produce a high nitrogen atom flux and thus a high growth rate. The Group III species is supplied as an organometallic compound via a gas injector which is disposed between the source of atomic nitrogen and the substrate manipulator, and which is thus nearer to the latter than the atomic nitrogen source.
It is an object of the present invention to provide an improved method of epitaxially growing a Group III nitride material by molecular beam epitaxy.